Tappet for a valve mechanism of an internal combustion engine

ABSTRACT

A tappet ( 1 ) with a hydraulic clearance compensation element ( 7 ), a positive opening of a one-way valve ( 15 ) being effected during a base circle region α of the contacting cam ( 5 ) whereby a closing member ( 16 ) of the one-way valve ( 15 ) is loaded by an adjusting element ( 21 ), the adjusting element ( 21 ) extends through a bottom ( 3 ) of the tappet ( 1 ) and cooperates with a groove ( 22 ) on the outer peripheral surface ( 23 ) of the cam ( 5 ), the groove ( 22 ) extends over the entire periphery of the cam ( 5 ) with the exception of its base circle region so that when the base circle region α comes to be situated opposite the bottom ( 3 ), a positive opening of the closing member ( 16 ) is effected whereby no undesired relieving of the gas exchange valve actuated by the tappet ( 1 ) nor its opening due to a transmission of undesired opening movements from the cam ( 5 ) to the clearance compensation element ( 7 ) can occur in the base circle region α of the cam ( 5 ).

FIELD OF THE INVENTION

The invention concerns a tappet for a valve train of an internalcombustion engine according to the features of the preamble of claim 1.

BACKGROUND OF THE INVENTION

A tappet of the pre-cited type is disclosed in DE-OS 41 11 610. Anadjusting element extends through a bottom of the tappet and acts at oneend on a closing member of a one-way valve of a clearance compensationelement and cooperates at the other end with a groove on the outerperipheral surface of a contacting cam. This groove is arranged only ona small part of the outer peripheral surface of the cam and extends onboth sides of the cam tip in the region of a lift-producing run-on andrun-off flank of the cam. By an opening of the closing member outside ofthe region of the groove of the cam, it is intended to achieve anydesired timing for valve intake and discharge. A more particular goal isto obtain an automatic shifting of the point of time of closing of thegas exchange valve concerned as a function of the engine speed.

However, this prior art document gives no incentive to the personskilled in the art as to how to relieve a high pressure chamber of theclearance compensation element during the cam base circle phase with thedisclosed means in order to protect the gas exchange valve from camshaftdisplacements, torsional vibrations of the camshaft and other factors ofinfluence such as circularity errors of the cam base circle which in themost unfavorable of cases can lead to the opening of the gas exchangevalve concerned during the base circle phase.

OBJECT OF THE INVENTION

The object of the invention is therefore to create a tappet of thepre-cited type in which the aforesaid drawbacks are eliminated and, inparticular, an undesired opening of its gas exchange valve during a basecircle phase is avoided by the use of simple means.

SUMMARY OF THE INVENTION

The novel tappet of the invention for a valve train of an internalcombustion engine, which tappet oscillates with its skirt in a bore of acylinder head and is contacted on a top surface of its bottom by a camof a camshaft, a hydraulic clearance compensation element being fixedagainst an undersurface of the bottom by a bushing which is in contactby one of its peripheral surfaces with a pressure piston that is axiallydisplaceable relative to the bushing, a bottom of the pressure pistonfaces at least one gas exchange valve, a high pressure chamber (13) isarranged axially between the bottom and an annular shoulder of thebushing and a reservoir for hydraulic medium is arranged preferablyaxially between the annular shoulder and the bottom, the annularshoulder having at least one passage which is closed on a side nearerthe high pressure chamber by a one-way valve which opens towards thehigh pressure chamber, a closing member of the one-way valve is biasedtowards the reservoir by a spring means, and the closing member can bedisplaced toward the bottom by an adjusting element which extendsthrough an opening of the bottom and cooperates with a groove on theouter peripheral surface of the cam, is characterized in that

the groove extends over an entire outer peripheral surface of the camwith the exception of its base circle region α, and the groove has adepth so that the adjusting element extending in the groove releases theclosing member so that the closing member bears against the passage dueto the force of its spring means, and

the force of the spring means has a magnitude so that for opening theclosing member in the base circle phase of the cam, a hydraulic mediumdifferential pressure of Δp≧0.4 bar is required at the cross-section Aof the passage.

Due to the fact that the groove of the cam extends over the entire outerperipheral surface thereof with the exception of the base circle region,the one-way valve of the clearance compensation element is defined andpositively opened. Due to this “opening”, the high pressure chamber anda reservoir of the clearance compensation element are in hydrauliccommunication with each other. As known, per se, to the person skilledin the art, possible displacements or torsional vibrations of thecamshaft and circularity errors in the cam base circle no longer lead toan undesired stiffening of the high pressure chamber and an eventualrelieving of the gas exchange valve loaded in closing direction, or evento an opening of the gas exchange valve. The invention, however,likewise covers solutions in which the groove of the cam also extends inparts of its base circle region.

During the aforementioned positive opening, clearance formed during thelifting phase of the gas exchange valve can be compensated in a knownmanner by a regulation of the quantity of hydraulic medium. At the sametime, an undesired build-up of high pressure in the high pressurechamber during the base circle phase is prevented. Retroaction ofadditional cams, such as cams of fuel injection pumps, on the hydraulicclearance compensation during the base circle phase of the camshaft islikewise prevented. It is further known that relatively largedilatations of the exhaust valve occur particularly in transientoperation, i.e. during a rapid acceleration of the internal combustionengine immediately upon its start. If it comes to the worst, it is alsopossible that the clearance compensation element cannot compensate forthese length variations by its leak function and the gas exchange valveremains partly open during the base circle of the cam. This drawback,too, can be eliminated in a simple manner by the means provided by theinvention.

According to a further feature of the main claim, the spring force ofthe spring means acting on the closing member is designed to besubstantially larger than that of prior art spring means. In this way,the sealing action of the one-way valve is clearly enhanced also, andmore particularly, at rapid accelerations of the tappet. It has beendetermined that no minimum hydraulic medium pressure from the reservoiris required for opening the closing member against the force of thespring means. Since this minimum pressure in prior art cup tappets isrelatively low, for example in the idle running range, the force of thespring means for the one-way valve also had to be relatively small inorder to assure a proper clearance compensating function of theclearance compensation element even in the idle running range. Theproposed higher spring force of the spring means also assures a fasterand more reliable closing of the one-way valve when cam lift starts. Atthe same time, it is also possible to realize a clearly larger stroke ofthe closing member compared to the prior art strokes so that,immediately upon a starting of the base circle phase, a relatively largeflow cross-section is established on the valve by the adjusting element.At the same time, the hitherto required relatively close tolerances withregard to the stroke of the closing member can be relaxed and this isparticularly favorable from the manufacturing point of view.

The proposed relatively high spring force of the spring means for theclosing member necessitates a hydraulic medium differential pressure ofΔp≧0.4 bar for opening the closing member in the base circle phase ofthe cam, while, due to the just described situation, the maximumdifferential pressures realized in the prior art were ≦0.2 bar. Thispressure is measured at the passage for the closing member but withouttaking into account the mass forces and other factors of influence whichneed no further specification here. Thus, the larger the force of thespring means acting on the closing member relative to the dimensions ofthe tappet, the more reliable is the closing behavior of the closingmember.

It is also conceivable to extend the non-recessed portion of the outerperipheral surface of the cam beyond the base circle region into thepre-cam region.

It is likewise possible to shorten a region of the closing ramp of thecam adjoining a run-off flank of the cam (seen in rotating direction ofthe cam). This is enabled by the measures (positive opening of theone-way valve) provided by the invention because the length of theclosing ramp can now be chosen without regard to different sinkingvalues of the clearance compensation element. In this way, theperformance or speed of rotation of the internal combustion engine canbe favorably influenced because a larger valve opening cross-section isascertained against time and a period of overlap with an opening ramp ofan oppositely acting gas exchange valve of the cylinder is shortened.Prior art closing ramps have a height of approximately 0.1 to 0.2 mmwhile, according to the invention, the closing ramp has a height of <0.1mm. Analogous to the height of the opening ramp, this height of <0.1 mmproves to be still necessary for eliminating the compressibility of theclearance compensation element.

According to a further feature of the invention, the spring means is asimple coiled spring and the one-way valve is a ball valve, known, perse. However, it is also conceivable to use other valves like disk valvesand the like. The spring means may further be a conical coil spring, adisk spring or any other similar suitable means.

It is particularly advantageous if the opening in the region of thebottom of the cup tappet at the same time comprises an air ventcross-section. In this way, air present in the reservoir can beevacuated in a simple manner through the air vent cross-section. Withthis cross-section, it is possible at the same time to realize a certainlubrication of the top surface of the bottom during the run-on phase ofthe cam.

Due to the positive opening of the closing member in the region of thebase circle of the cam, air which might be present in the high pressurechamber can also escape to the outside. Thus, it is no longer necessaryto arrange the reservoir above the high pressure chamber in axialdirection for collecting the escaping air there. This offers a furtherpossibility of minimizing an assembly length between the bottom of thepressure piston and the bottom of the tappet.

Particularly favorable flow conditions are created on the adjustingelement in that this has a conical shape starting from the closingmember. It is also possible to provide a bevel on an end face of theadjusting element facing the closing member, so that favorable flowconditions for the hydraulic medium are also created on this bevel.

According to a further characterizing feature of the invention, theadjusting element is provided with a securing element such as a diskspring. This disk spring prevents an undesired falling-out of theadjusting element from the interior of the tappet particularly duringtransportation of the tappet. This spring further prevents undefinedaxial movements of the adjusting element. Many different types ofsecuring elements may be used, even such as are made in one piece withthe adjusting element and other similar securing elements.

It is to be understood, that the invention relates to a variety ofhydraulic cam followers and not only to the cup-shaped tappet describedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described more closely with reference to thedrawings which show:

FIG. 1, a partial longitudinal section through a tappet according to theinvention, and

FIG. 2, a schematic representation of a cam comprising a grooveaccording to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a tappet 1 whose function does not need furtherspecification in the present context. This tappet 1 comprises a hollowcylindrical skirt 2 with which the tappet 1 oscillates in a bore of acylinder head of an internal combustion engine (not shown). The skirt 2is closed at one end by a bottom 3. A top surface 4 of the bottom 3 iscontacted in lift direction by a cam 5 provided with a closing ramp (B)adjoining a run-off flank (8) (see also FIG. 2).

A hydraulic clearance compensation element 7 bears centrally against anundersurface 6 of the bottom 3. This clearance compensation element 7comprises a bushing 8 which extends directly on the undersurface 6. Aperipheral surface 9 (outer peripheral surface) of the bushing 8 ispartially enclosed by a pressure piston 10. This pressure piston 10comprises a bottom 11 facing away from the bottom 3. Through this bottom11, the entire clearance compensation element 7 and thus also the tappet1 is supported on a gas exchange valve, not shown.

Axially between a radially inward directed annular shoulder 12 of thebushing 8 and the bottom 11, there is arranged a high pressure chamber13 for hydraulic medium. A reservoir 14 for hydraulic medium is arrangedabove the annular shoulder 13, between this annular shoulder 13 and thebottom 3.

A one-way valve 15 comprising a closing member 16 in the form of a balland a spring means 17 is arranged in the high pressure chamber 13. Thespring means 17 is supported at one end on a retaining cap 18 and actsat the other end on the closing member 16 in the direction of the bottom3. In the view of FIG. 1, the closing member 16 closes a passage 19 onthe annular shoulder 12. The passage 19 has a cross-section A.

At the same time, an adjusting element 21 configured as a pin extendsthrough a central opening 20 in the bottom 3. This adjusting element 21acts at one end on the closing member 16 and projects at the other endbeyond the top surface 4 of the bottom 3 toward the cam 5. The cam 5, inits turn, possesses, on its outer peripheral surface 23, a groove 22facing the adjusting element 21. This groove 22 extends outside of abase circle region α of the cam 5 over the entire periphery thereof. Thegroove 22 has a depth related to a length of the adjusting element 21 sothat, when the groove 22 is situated opposite the adjusting element 21,the adjusting element 21 relieves the closing member 16 so that thiscompletely closes the passage 19 which is preferably made as a bore. Onthe other hand, with a running-on of the base circle region α of the cam5, the adjusting element 21 is positively displaced toward the bottom 11of the pressure piston 10 so that a sufficiently large flowcross-section for the hydraulic medium is formed between the closingmember 16 and the passage 19. Thus, on the one hand, the quantity ofhydraulic medium pressed out of the high pressure chamber 13 during thelift phase of the cam 5 for effecting clearance compensation can bere-sucked in a simple manner into the high pressure chamber 13. On theother hand, due to this positive opening, torsional vibrations of thecamshaft, circularity errors of the cam and other factors of influence(see also the introduction of the description) no longer have adetrimental effect on the clearance compensating function of theclearance compensation element 7.

Due to the fact that the closing member 16 leaves the passage 19 openduring the entire base circle region a, an undesired stiffening of thehigh pressure chamber 13 can no longer occur and thus also no relievingof the gas exchange valve or even opening of the gas exchange valve withits known drawbacks is encountered. Moreover, the spring means 17 nolonger needs to be compressed via the closing member 16 by the hydraulicmedium pressure provided from the reservoir 14 at the cross-section A.As a result, the force of the spring means 17 can be very much higherthan the forces of prior art spring means.

A Belleville-type spring washer 25 is arranged within the reservoir 14on an outer peripheral surface 24 of the adjusting element 21. Thiswasher 25, while permitting an axial movement of the adjusting element21 toward the bottom 11, prevents the adjusting element 21 from fallingout of its opening 20.

What is claimed is:
 1. A valve train of an internal combustion enginecomprising a tappet (1) loaded by a cam (5) of a camshaft, which tappet(1) oscillates with a skirt (2) in a bore of a cylinder head and iscontacted on a top surface (4) of a bottom (3) by the cam (5), ahydraulic clearance compensation element (7) being fixed against anundersurface (6) of the bottom (3) by a bushing (8) which is in contactby one of the bushing's peripheral surfaces (9) with a pressure piston(10) that is axially displaceable relative to the bushing (8), a bottom(11) of the pressure piston (10) faces at least one gas exchange valve,a high pressure chamber (13) is arranged axially between the bottom (11)and an annular shoulder (12) of the bushing (8) and a reservoir (14) forhydraulic medium is, arranged preferably axially between the annularshoulder (12) and the bottom (3), the annular shoulder (12) having atleast one passage (19) which is closed on a side nearer the highpressure chamber (13) by a one-way valve (15) which opens towards thehigh pressure chamber (13), a closing member (16) of the one-way valve(15) is biased towards the reservoir (14) by a spring means (17), andthe closing member (16) can be displaced toward the bottom (11) by anadjusting element (21) which extends through an opening (20) of thebottom (3) and cooperates with a groove (22) on the outer peripheralsurface (23) of the cam (5), characterized in that the groove (22)extends over the entire outer peripheral surface (23) of the cam (5)with the exception of a base circle region α, and the groove (22) has adepth so that the adjusting element (21) extending in the groove (22)releases the closing member (16) so that the closing member (16) bearsagainst the passage (19) due to the force of the spring means (17); theforce of the spring means (17) has a magnitude so that for opening theclosing member (16) during contact of the base circle region α of thecam (5), a hydraulic medium differential pressure of Δp≧0.4 bar isrequired at the cross-section A of the passage (19); and a closing ramp(β) adjoining a run-off flank (γ) of the cam (5), as seen in rotatingdirection of the cam (5), has a height of <0.1 mm.
 2. A valve train ofan internal combustion engine with a means for avoiding high pressure ina high pressure chamber of a tappet during base circle operation of theengine, comprising a tappet (1) loaded by a cam (5) of a camshaft, whichtappet (1) oscillates with a skirt (2) in a bore of a cylinder head andis contacted on a top surface (4) of a bottom (3) by the cam (5), ahydraulic clearance compensation element (7) being fixed against anundersurface (6) of the bottom (3) by a bushing (8) which is in contactby one of the bushing's peripheral surfaces (9) with a pressure piston(10) that is axially displaceable relative to the bushing (8), a bottom(11) of the pressure piston (10) faces at least one gas exchange valve,a high pressure chamber (13) is arranged axially between the bottom (11)and an annular shoulder (12) of the bushing (8) and a reservoir (14) forhydraulic medium is arranged preferably axially between the annularshoulder (12) and the bottom (3), the annular shoulder (12) having atleast one passage (19) which is closed on a side nearer the highpressure chamber (13) by a one-way valve (15) which opens towards thehigh pressure chamber (13), a closing member (16) of the one-way valve(15) is biased towards the reservoir (14) by a spring means (17), andthe closing member (16) can be displaced toward the bottom (11) by anadjusting element (21) which extends through an opening (20) of thebottom (3) and cooperates with a groove (22) on the outer peripheralsurface (23) of the cam (5), wherein the groove (22) extends over theentire outer peripheral surface (23) of the cam (5) with the exceptionof a base circle region α, and the groove (22) has a depth so that theadjusting element (21) extending in the groove (22) releases the closingmember (16) so that the closing member (16) bears against the passage(19) due to the force of the spring means (17), the force of the springmeans (17) has a magnitude so that for opening the closing member (16)during contact of the base circle region α of the cam (5), a hydraulicmedium differential pressure of Δp≧0.4 bar is required at thecross-section A of the passage (19); and a closing ramp (β) adjoining arun-off flank (γ) of the cam (5), as seen in rotating direction of thecam (5), has a height of <0.1 mm.
 3. A tappet according to claim 1,characterized in that the spring means (17) is a coiled spring and theone-way valve (15) is a ball valve.
 4. A tappet according to claim 1,characterized in that the opening (20) is simultaneously configured asan air vent for the reservoir (14).
 5. A tappet according to claim 1,characterized in that, starting from a valve-proximate end, theadjusting element (21) has a conical shape, a partly conical shape, oran increasing diameter.
 6. A tappet according to claim 1, characterizedin that the adjusting element (21) is a pin or a pin-like element.
 7. Atappet according to claim 1, characterized in that the adjusting member(21) is connected at its outer peripheral surface (24) within thereservoir (14) to a securing element (25).
 8. A tappet according toclaim 7, characterized in that the securing element (25) is a springwhich is supported on the undersurface (6) of the bottom (3), and theforce of this spring is clearly smaller than the force of the springmeans (17).
 9. A tappet according to claim 7, characterized in that thesecuring element (25) is made as a Belleville-type spring washer or as adisk.